Method for clock rate conversion

ABSTRACT

A method for clock rate conversion of digitized television signals for the acquisition of a television signal having a prescribed processing clock rate is provided wherein the digitized television signal is band-limited, a clock rate conversion being subsequently undertaken. The television signal having a second clock rate is written into a memory 6 and is, in turn, read with the prescribed processing clock rate, whereby an unaltering plurality of data words corresponding to picture points is not output in every television line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for clock rate conversion ofdigitized television signals for generating a television signal having aprescribed processing clock rate given a fixed scan clock rate of thecoded television signal.

2. Description of the Prior Art

The clock rates for sampling an analog television signal and the clockrate for further processing or, respectively, transmission of thedigitized television signal are rigidly prescribed in the transmissionof black-and-white and color television signals. These clock ratesfrequently do not coincide and likewise do not have a rationalrelationship. A clock rate conversion by which television signals havingprescribed processing clock rates are acquired must then occur. Thesetelevision signals can be supplied to a corresponding coder for furtherdata reduction by way of a DPCM method.

What is meant by television signals are both the luminance signals andthe chrominance signals. At present, the method is of interestparticularly for luminance signals.

Various methods for clock rate conversion are known. A data streamhaving a first clock frequency is thereby converted into a second datastream having a different clock frequency. Such a method is disclosed inthe periodical "Frequenz", Vol. 36, No. 10, 1982, pp. 275-279. To thisend, a time-invariant, linear filter is employed to which the digitizedsignal values are supplied with the first clock frequency and whichemits the digitized signals with a second clock frequency at the outputvia a summer element. This conversion method is co-employed in thepresent invention. The method, however, presumes that the relationshipof the second clock rate to the first clock rate yields a rationalvalue. Given the processing clock rate that are currently standard,however, this is usually not the case.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide a clockrate conversion method for digitized television signals which enablesthe acquisition of the television signal with a prescribed processingclock rate, even given a non-rational clock ratio.

Proceeding from the art mentioned above, the object of the presentinvention is achieved in that the television signal is converted by wayof clock rate conversion with a conversion factor representing arational number, being converted into a second television signal havinga second clock rate, in that the data words of the second televisionsignal are written into a memory with the second clock rate, and inthat, of the stored data words of each television line, a plurality ofdata words deviating therefrom is read with the processing clock rate.

The extremely difficult problem of clock rate conversion givennon-rational clock relationships is resolved in a simple manner by wayof the present invention by the emission or addition of a few (m)digital signal values (picture points). The luminance signal, or achrominance signal as well, is generally referred to here as atelevision signal. This is achieved via a memory in which, for example,all picture points of a television line are written as data words, but afew picture points at the beginning and at the end of each televisionline are not transmitted. The writing into the memory occurs with thesecond clock frequency generated by the scan rate converted and thereading of the memory occurs with the prescribed processing clock rate.

The realization of such a method presents no difficulties and requiresonly a slight expense. The omission of a few picture points can, ofcourse, occur upon writing into the memory, as well as upon reading fromthe memory. The slight distortion of the read television pictures canhardly be noticed. It can, further, be compensated by means of acorresponding adjustment of the receiving apparatus.

It is advantageous in that the second television signal isintermediately stored in a first-in/first-out memory.

In terms of expense, the method can be most favorably implemented byusing a first-in/first-out (FIFO) memory.

It is expedient that 13.5 MHz be provided as the scan rate, 10.125 MHzas the second clock rate and 3/4 as the conversion factor.

The conversion of the digitized television signal having a clock rate of13.5 MHz into a digital television signal having a second clockfrequency of 10.125 MHz is expedient since only a simple conversionfactor of 3/4 need be realized.

It is advantageous that the first three and the last three data words ofrespectively 526 data words of a television line of the secondtelevision signal not be read.

The television picture is only slightly changed due to the omission ofthese data words.

BRIEF DESCRIPTION OF THE DRAWING

Other objects, features and advantages of the invention, itsorganization, construction and operation will be best understood fromthe following detailed description, taken on conjunction with theaccompanying drawing, on which:

FIG. 1 is a basic block circuit diagram for clock rate conversionaccording to the present invention; and

FIG. 2 is a schematic representation of an excerpt from a televisionpicture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawing, an analog television signal AS is supplied toa separation circuit 1 via an input 1₁. A first output 1₂ of theseparation circuit 1 is connected to the input of an analog/digitalconverter 2 which also contains a sample-and-hold circuit. A secondoutput 1₃ of the separation circuit 1 is connected to an input 5₁ of aclock generator 5. The output of the analog/digital converter 2 isconnected to an input 3₁ of a digital low-pass filter 3 which has anoutput 3₃ connected to the data input 4₁ of a clock rate converter 4which, in turn, has an output 4₄ connected to the data input 6₁ of amemory constructed as a first-in/first-out (FIFO) memory. The dataoutput of the memory is referenced 6₄. For example, the clock generator5 contains a plurality of phase-locked loops (PLL) which serve forgenerating the working clocks with the frequencies f_(T1) , f_(T2) andf_(T3). At an output 5₂, the sampling frequency f_(T1) is supplied tothe analog/digital converter 2 and the digital low-pass filter 3 via itsclock input 3₂. The same frequency is supplied to the clock rateconverter 4 via its clock input 4₂. A second clock frequency f_(T2) isemitted at the output 5₃ of the clock generator 5 and is supplied to theclock rate converter 4 as a read-out clock via a second clock input 4₃,and to the memory 6 via its first clock input 6₂. The processing clockrate f_(T3) is supplied from a third clock output 5₄ of the clockgenerator to the second clock input 6₃ of the memory 6. Synchronizingpulses SI are also output at the output 5₅ of the clock generator.

In this exemplary embodiment, a luminance signal which is referred togenerally as a television signal in the following text is acquired froma closed-coder color television signal FS.

A digitized color television signal FS is acquired from the analogtelevision signal AS via the analog/digital converter 2. The televisionsignal LU1 is filtered out of the color television signal by thelow-pass filter 3 whose limit frequency lies at about 3.9 MHz. The clockrate converter 4 converts the television signal LU1 into a secondtelevision signal LU2 whose frequency lies close to the processingfrequency f_(T3). When the sampling of the analog television signal ASoccurred with 13.5 MHz, then the television signal LU1 at the output ofthe digital low-pass filter 3 has the same data word rate. The data wordrate is converted to 10.125 MHz by way of the clock rate converter 4.The conversion factor C of the clock rate converter 4 thereby amounts to3/4. The data words are written into the memory 6 with a clock havingthe same frequency f_(T2). The reading occurs with a prescribedprocessing clock 10 MHz. The first three (m/2) and the last three datawords of the luminance signal LU2 are respectively not written given useof a FIFO memory and, therefore, are not read (FIG. 2). The second clockfrequency f_(T2) is synchronized with the processing clock rate f_(T3)via a phase-locked loop. The line sync pulses that are contained in thesynchronizing pulses SI are employed for this purpose. A simplesuppression of the write operation is also possible by way thereof byinhibiting the clock used for writing.

The synchronizing pulses SI are generally output separately from thetelevision signal 3 read from the memory 6.

Although we have described our invention by reference to particularillustrative embodiments thereof, many changes and modifications of theinvention may become apparent to those skilled in the art withoutdeparting from the spirit and scope of the invention. We thereforeintend to include within the patent warranted hereon all such changesand modifications as may reasonably and properly be included within thescope of our contribution to the art.

We claim:
 1. A clock rate conversion method for first digitizedtelevision signals of television lines sampled at a first clock rateinto second television signals having a predetermined processing clockrate, comprising the steps of:generating clock signals having a firstclock rate (f_(T1)), a second clock rate (f_(T2)) and, as thepredetermined processing clock rate, a third clock rate (f_(T3)), wherethe second clock rate is related to the first clock rate by a rationalnumber conversion factor (C) and is close to the third clock rate;sampling the first digitized television signals at the first clock rate(f_(T1)) to obtain data words; converting the data words to the secondclock rate (f_(T2)) and storing the same; and reading a plurality ofdata words (K±m) of K data words of each television line at the thirdclock rate (f_(T3)).
 2. The method of claim 1, wherein the step ofstoring is further defined as:storing the data words on afirst-in/first-out basis.
 3. The method of claim 1, wherein the step ofgenerating clock signals is further defined as:generating the firstclock rate (f_(T1)) at 13.5 MHz; and generating the second clock rate at10.125 MHz with a conversion factor (C) of 3/4.
 4. The method of claim1, wherein the step of storing and reading are further definedas:storing 526 data words for each television line on afirst-in/first-out basis; and reading all of the data words for eachline except the first three data words and the last three data words. 5.The method of claim 1, wherein the step of reading is further definedas:reading more data words than stored for each television line as ablack or gray luminance value.
 6. The method of claim 1, and furthercomprising the step of:filtering the sampled first digitized televisionsignals with a digital low-pass filter.
 7. The method of claim 1,wherein the step of generating clock signals is further definedas:generating the clock signals with at least one phase-locked loop. 8.The method of claim 1, and further comprising the step of:selecting theconversion factor (C) to have the lowest possible numerator and thelowest possible denominator.